From here, select “Change Plan Settings” next to whichever power plan you are currently using.

This will show what your current settings are for brightness, turn off the display, and sleep while the unit is both plugged in and running on battery.

If, under the “On battery” column, the brightness is lower than the brightness setting for “Plugged in”, this would mean that the brightness is automatically reduced to that level when you disconnect the power cord.

If you wish to prevent this from occurring, ensure that the brightness levels in both columns match.

Don't forget to click "Save changes' once you have made any adjustments.